This invention relates to an improvement in the prior art process for void-free packaging of high density thick foil (HDTF) circuitry, and to the utilization therein, as a new article of manufacture, of a laminated board having thermoformed therein a precisely shaped and dimensioned cavity. The lateral walls of the cavity reproduce faithfully a convex profile, approximating in cross-section a circular arc complementary to the characteristic concavely dished profile of the lateral wall of a chemically milled surface. In such a preformed cavity the circuitry is rigidly confined within its walls in an intimately mating relationship. In addition to being immobilized, it is protected from corrosion by virtue of the elimination of voids which can provide and maintain a corrosive environment, notably moisture, among other elements.
HDTF circuitry is a relatively recent development as compared with low current (LC) circuitry. In LC circuitry, where the continuous duty rating never exceeds 0.5 amperes, the conductor is thin, i.e., it has a thickness of from about 0.0007 inch to about 0.0028 inch.
LC circuitry is satisfactorily packaged by the process of the prior art, i.e., the press and curing cycle specified by the manufacturer of the prepreg (resin-imbedded woven glass fabric) for the specific resin involved, whereby the circuitry, adhesively supported on a supporting resin layer, is itself superimposed over a bonding sheet overlaid upon the precursor of the laminated board, i.e., a plurality of layers of prepreg. For a prepreg based on epoxy resin this stacked composite is placed between the platens of a press preheated to 340.degree. F. and held therebetween for a low pressure dwell at 20 psi for 180 seconds followed by a high pressure dwell at 250 psi for 57 minutes to complete the necessary 60 mins. at 340.degree. F. for completing the cure of the resin of the precursor. The symbol "psi" as used throughout this disclosure adopts a commonly used designation for pressure in lbs./in..sup.2 based on the net plan view area of the contacting surface.
For a laminated board based on polyimide resin the press and curing cycle specified by the manufacturer is: preheat the platens of the press to 250.degree. F., insert the stacked composite therebetween, apply 200-500 psi pressure for 15 minutes, raise the temperature to 350.degree. F. and hold under pressure for 2 hours, cool platens to 125.degree. F., release the pressure, remove the stacked composite from the press, separate the uncured laminated circuit package from the other components of the stacked composite, reclamp and place into a circulating air oven at 425.degree. F. for 4 hours to effect a complete cure.
The epoxy or polyimide resin composite, after having achieved a complete cure at high temperature, is cooled to ambient temperature under pressure, released from pressure, and the completed laminated circuit package is removed from the press or clamp, respectively.
At the high temperature in the press the bonding sheet softens and flows under pressure, being retained only in minimal amount as adhesive film for bonding the package components together. Last but most importantly, the circuitry, backed by the supporting sheet which is already in the completely cured stage and therefore unyielding, is pushed into the heat-softened resin of the precursor to form its own cavity therein.
The precursor of the laminated board, as used herein, consists of a stacked plurality of individual sheets of resin-imbedded woven glass fabric, commonly known as "prepreg." The resin of prepreg is a thermosetting resin in an incomplete state of cure, generally referred to as "B stage."
The above described process for LC circuitry is unsatisfactory for HDTF circuitry, where the conductor thickness of from about 0.004 inch to about 0.025 inch, typically about 0.010 inch, is much heavier in order to carry a much higher continuous duty rating of from about 5 A. to about 100 A., typically about 40 A. A correspondingly deeper cavity must be formed in the precursor. The ratio of the depth of the cavity to the overall pressed thickness of the laminated board containing said cavity, hereinafter designated as the compression ratio (CR), typically ranges from about 35% to about 77% for HDTF circuitry as compared with about 25% max. for LC circuitry.
In the packaging of HDTF circuitry in comparison with LC circuitry the displacement of resin is greater, while the downward distortion and compression and the horizontal stretching of the woven glass fabric is more severe. These factors combine to aggravate the problem of proper flow of the heat-softened prepreg resin in the press to obtain good distribution and intimate packing thereof around the circuitry as it sinks into the resin. The thermoplastic resin bonding the circuitry to the supporting sheet likewise softens at the temperature of the press, freeing the circuitry to swim and drift out of its intended position and from its precise registry with associated components of circuitry. Large air bubbles or voids occur adjacent the newly formed cavity thereby rendering it oversize and incapable of rigidly securing the circuitry. These bubbles provide a site favorable for a corrosive environment, possibly even a tunnel for ingress of corrosive elements, notably moisture, from the exterior.
Other alternative methods for forming the cavity in a laminated board are:
a. mechanical milling PA1 b. chemical milling, i.e., photoetching
Either one of these has the disadvantage of removing one or more layers of glass fabric in the laminated board, thereby structurally weakening it. The lateral walls of the cavity formed by mechanical milling are rectalinearly vertical; by chemical milling they are concavely dished, with a profile as seen in cross-section of an approximately circular arc. Neither of these profiles properly matches that of the HDTF circuitry in an intimately mated relationship. Typically, the circuitry is formed by chemical milling and hence also has a concavely dished lateral wall seen in cross-section as an approximately circular arc, which cannot properly register with the above mentioned rectalinearly vertical or concavely dished walls of the preformed cavity.
We have found that a cavity that will properly receive and intimately confine HDTF circuitry produced by photomasking a blank from a pattern master transparency followed by chemical milling in the unmasked portions is formed by a form tool which likewise is produced by photomasking a metal plate from the same pattern master transparency followed by chemical milling in the unmasked portions to a depth equal to the thickness of the circuitry. This process, hereinafter referred to as photoetching, faithfully reproduces the characteristic profile of chemical milling, as well as the unique design of the master pattern and its dimensions.
One of the objects of the invention is to thermoform a cavity in a resin-imbedded woven glass fabric laminated board, faithfully repreducing the pattern shape, depth, profile and dimensions of HDTF circuitry, i.e., including convex lateral walls complementary to the concave profile of the lateral walls of the circuitry.
Another object of the invention is to thermoform a cavity in a laminated board free of air bubbles adjacent to the cavity.
A third object of the invention is to thermoform a cavity with a compression ratio of from about 35% to about 77% in a laminated board without destroying the integrity of one or more fabric layers.
A fourth object of the invention is to preform a cavity in a laminated board with minimum XY (lateral) flow of the imbedding resin.
A fifth object of the invention is to vertically depress and compress, and to horizontally stretch the glass fabric so as to cradle the cavity in the nested plurality of woven glass fabrics in the laminated board.
A sixth object of the invention is to provide a laminated board with a cavity therein cradled in the nested plurality of woven glass fabrics which remain under residual tension at ambient temperature, thereby maintaining compressive forces on the resin in which they are imbedded.
A seventh object of the invention is to provide a void-free laminated package of thick film circuitry.